Weather Conditions

The number of clear nights (zero cloudiness) is about 170 a year. The number of useful nights is about 240. The best season, when practically no clouds are observed, is June to August, while the highest chance for clouds are in the period January to April. Winds are usually moderate mainly from North-East and North. Storm wind velocities (greater than 40 km/h) occur, but rarely. The wind speed tends to decrease during the night. Temperature gradients are small and fairly moderate. The average relative humidity is quite high with a tendency to decline during the night from April to August.

The average seeing is about 2-3 seconds of arc. A few good nights have seeing of 1" or less while some show seeing larger than 5".

Typical extinction coefficients, in mag/airmass, are:

kV=0.24

kU-B=0.22

kB-V=0.14

kV-R=0.05

kR-I=0.07

These are median values for the last decade.

Further details from a paper: "Observing Conditions at the Wise Observatory", by Dr. Noah Brosch:

The quality of the nights at the Wise Observatory, in terms of cloudiness and fraction of useful time, has been collected from the interim observing reports filled by the astronomers at the end of each observing night for 17 consecutive years. This report is based on 11 years of operating the observatory that were processed manually and six years when the interim reports were processed by computer. The fraction of nights when no clouds were reported is typically about 70%. The best season, when practically no clouds are observed, is June to August, while the highest chance for clouds are in the period January to April. The actual breakdown of cloudiness percentage for individual months is given in Table A.1.

Table A.1: Clear nights 1972 to 1983

Month

Percent clear

January

52 [10]

February

56 [17]

March

55 [12]

April

51 [10]

May

77 [10]

June

89 [10]

July

92 [06]

August

90 [05]

September

76 [16]

October

67 [19]

November

60 [12]

December

64 [13]

Yearly mean

69 [15]

Note: The standard deviation of the mean is given in square brackets.

A.1 Extinction

Since 1975 extinction coefficients are measured at the Wise Observatory on a fairly regular basis. Their collection gives an idea about the character of the site and may be useful to observers if no measurement of the extinction was obtained for a certain observation. The results presented here combine about 100 observing nights when at least UBV standard photometry with adequate standards was done. We include among them results presented by Vidal et al. (1978) from the preliminary test of the photometric qualities of the site. The photoelectric system used at the Wise Observatory is based on Landolt (1973) UBV standards with the RI extension based on stars from Moffett and Barnes (1979). Table A.2 shows the median value of the extinction coefficients for different years and the number of different nights from which the median values were derived.

Table A.2: Median extinction coefficients for the Wise Observatory

Period

KV

KB-V

KU-B

KV-R

KR-I

Nmeasurements

1975-1980

0.26

0.15

0.33

-

-

5

1981-1982

0.26

0.17

0.31

-

-

20

1984

0/45

0.07

0.14

0.01

0.02

9

1986-1987

0.24

0.14

0.23

0.05

0.06

23

1988

0.24

0.14

0.22

0.05

0.07

26

1989

0.22

0.14

0.24

0.08

0.04

14

Notes: The measurements before 1984 used the bi-alkali photomultiplier and were limited to the UBV bands. We find no ready explanation for the anomalous extinction coefficients measured in 1984.

The typical extinction at zenith, taken as the median value over all the measurements, is 0.24 at V. The color-dependent terms are kB-V=0.14, kU-B=0.22, kV-R=0.05 and kR-I=0.07.
For comparison, the typical extinction at La Silla, as given in the ESO User's Manual, is 0.11 at V and the color-dependent terms are kB-V=0.09, kU-B=0.26, kV-R=0.08 and kR-I=0.02. Similar values can be derived for the CFHT. At Mauna Kea Krisciunas et al. (1987) measured kV=0.113 and kB-V=0.082 at the mountain peak and kV=0.149, kB-V=0.158 at 2800 m altitude.
The extinction at Wise is slightly worse than at ESO or at the CFHT, by about 0.13 mag at zenith. This is to be expected, considering the altitude difference between Wise Observatory and ESO/La Silla (2400m) or the CFHT (4204m).

A.2 Sky Brightness

The sky brightness is measured on a star-free sky patch one arcminute in diameter. In 1976 November-December the zenith sky brightness toward the Perseus cluster of galaxies yielded ``U''=23.0 (see note about the definition of the U-band in Vidal et al., 1978), B=22.7 and V=21.6 mag/square arcsec. In 1979 July the sky brightness during a dark night was measured at U=21.7, B=22.2 and V=21.6 mag./square arcsec. In March 1989 the measurement was repeated towards the Coma region with a different photomultiplier and filter combination. The results are comparable with those of 1979, implying no worsening of the sky conditions.

Table A.3: Sky brightness in mag/square arcsec

Period/Band

U

B

V

R

I

Notes

Nov-Dec 1976

23.0

22.7

21.6

-

-

frp, Vidal et al. (1978)

July 1979

21.7

22.2

21.6

-

-

Bi-li tube

Mar 1989

21.5

22.2

21.7

21.2

20.4

Ga-As tube

AAT

-

22.5

21.5

20.8

19.3

AAO Newsletter

CFHT

21.6

22.3

21.1

20.3

19.2

User's manual

ESO

22.0

23.0

21.9

21.1

20.2

User's manual

KPNO

-

22.9

21.9

-

-

Garstamg

CTIO

-

22.5

21.6

-

-

Garstamg

DDO

-

19.9

19.2

-

-

Garstamg

Palomar

-

22.9

21.5

-

-

Garstamg

A summary of sky brightness measurements is given in Table A.3, together with representative values for CFHT (CFHT User's manual, 1990 edition, p. 5-2), for ESO (ESO User's manual), and for the AAT (AAO Newsletter No. 56, 1991). A location in California, with dark skies, was reported to have a typical sky brightness of B=22.3-23.1 and V=21.2-22.1, depending on solar activity (Walker, 1988). From all these we conclude that Wise Observatory has a similar sky brightness as other observatories.

A.3 Seeing

The typical seeing reported by various observers at the Wise Observatory ranges from 2" to 3". Rare nights exhibit 1" seeing or less; these are only a few per year and occur mainly in winter, after the passage of a storm front. This determination is based on individual, visual estimates of various observers, sometimes by observing double stars with known separation, and is confirmed by measurements of stellar integrated profiles obtained with the photometer and a large aperture, while the star is drifting at the sidereal rate through the field of view.

The seeing evaluation is also based on many hundreds of CCD exposures obtained since 1986 that were analyzed at the Wise Observatory. The average FWHM of stellar images is 3.5". Bad seeing produces images of 7" FWHM; worse images are not recorded. In general, the seeing improves after midnight, following the reduced wind speed mentioned by Vidal and Feldman (1974).